tools: add pkg-diff script

Compare two list of packages and show the differences.
Expected syntax of file:
<package_name> <version>

Use:
diff-pkg old.packages new.packages

To generate the list of packages:
* Red Hat / CentOS: rpm -qa --queryformat '%{NAME} %{VERSION}'
* Debian / Ubuntu: dpkg -l | grep '^ii' | awk '{print $2 "\t" $3}'

Note: the package list generation is done by eDeploy in build/packages.
pkg-diff: Silencing grep to avoid miss-matching

The {new|delete}_pkg() functions are expected to print the matched
package. As the grep wasn't silenced, it was always reporting packages
as matching.

Silencing the grep make it work.
pkg_diff: Improving output

This patch format the output to ease the reading when comparing the version.
A fixed column approach is used, package name are left-aligned while version are right-aligned.

Note that if a package is new, a N/A is shown in the previous release.

Enjoy,

 ### Version changes ###
Package name                       Previous version             Newer version
------------------------- ------------------------- -------------------------
nova-api                                     2014.1                    2014.2
cinder-api                                      N/A                    2014.2
Merge pull request #112 from enovance/compare_pkg_list

tools: add pkg-diff script